The present invention relates to a magnetically shielding structure utilizing a superconductor cylinder and, in particular, it relates to a magnetically shielding structure in which magnetically shielding capability is made to improve by combining magnetically shielding materials of various different properties and in which a penetrating magnetic field from the opening portion of the superconductor cylinder is reduced to a very small value, the volume of a usable highly magnetically shielded space inside the cylinder bore is increased, or a very feeble magnetic field can be realized in the cylinder bore even in the case of a short superconductor cylinder. Thereby, a much feebler magnetic field than an external one can be effectively realized in the cylinder bore.
In a magnetically shielding structure constructed by a superconductor, the Meissner effect is utilized for the magnetic shield. That is, a material having the Meissner effect is, for example, formed into a cylinder shape to form a shielding body and is cooled below the critical temperature Tc for the transition to a superconductive state for making the shielding body a diamagnetic and, thereby, a magnetic flux is forced out to the exterior of the shielding body, and the internal space of the shielding body is magnetically shielded.
On the other hand, in the case of a shielding structure with a highly permeable material being commonly used without utilizing a superconductor, where a shielding body is formed into a cylinder with the highly permeable material, for example, and if the shielding body is held in a magnetic field, magnetic induction is generated in the wall of the shielding body and the magnetic field is short-circuited along the shielding body. The internal bore space of the cylinder is magnetically shielded thereby.
In such a magnetically shielding structure utilizing a superconductor, although the magnetically shielding capability of a cylindrical shielding body, for example, is high enough for a magnetic field parallel to the center axis of the cylinder (longitudinal magnetic field), the magnetically shielding capability for a magnetic field perpendicular to the center axis (lateral magnetic field) is not enough. Therefore, there is a problem in that the length of the cylinder has to be long in comparison with the inner diameter of the cylinder.
On the other hand, in the case of a cylindrical shielding body formed with a highly permeable material, for example, the shielding capability for longitudinal magnetic field is not enough in comparison with that for lateral magnetic field. Also, because of the limited value of permeability, the shielding efficiency of a single-layer cylinder can not be high enough, so that, for obtaining high shielding capability, a plurality of cylinder walls have to be laminated and a structure in which the inner layer is shorter than the outer layer has to be adopted. As a result, even in the case of a highly permeable material being used, there has been a problem in that the length of a cylinder outer layer becomes long. When a usable space is to be larger, the dimensions in the radial dirction and also in the axial direction have to be large, which causes a problem in that the shielding body becomes expensive.
The present invention is invented considering various aspects as described above, and the object of the invention is to provide a magnetically shielding structure in which the shielding capability is improved by combining magnetically shielding materials of various different properties. A volume of a highly usable magnetically shielded space is increased by suppressing a penetrating magnetic field through the opening portion of a superconductor cylinder to be at a low value. Alternatively, an extremely feeble magnetic field can be realized in a specified space in the cylinder even in the case of a short superconductor cylinder. Thereby an extremely feeble magnetic field can be efficiently realized in comparison with an external magnetic field.
According to the present invention, the above-mentioned object can be solved with a magnetically shielding structure having the features as described in each of the claims.
In a magnetically shielding structure according to the present invention, against a penetrating magnetic field, which shows an attenuating distribution toward the center on the longitudinal axis of a cylindrical shielding body composed of a material manifesting the Meissner effect, a wide variety of combinations of the cylindrical magnetic shielding body composed of a superconductive material, which manifests the Meissner effect, and a cylindrical collar means made of a highly permeable material having a through opening along the longitudinal direction of the cylindrical magnetic shielding body are disposed for absorbing and magnetically shorting the penetrating magnetic field by the magnetic induction generated in the highly permeable material.
FIG. 20 shows a schematic diagram of a penetrating magnetic vector inside a superconductor cylinder when a lateral magnetic field is applied to the superconductor cylinder. A penetrating magnetic field being thus distributed is attenuated to decrease the penetration quantity coming into the inside of the superconductor cylinder by utilizing a property of a highly permeable member, called magnetic induction, which magnetically shorts the penetrating magnetic field. Since a highly permeable member may have a residual magnetic field, it has to be disposed in a position where the residual magnetic field does not exert its magnetic influence on the objective space of a feeble magnetic field.
FIG. 1 is an illustrative representation showing the constitution of an ordinary embodiment of the present invention. In the figure, there is shown an end portion of a cylindrical shielding body (1) composed of an oxide superconductive material. The length of the shielding body (1) is generally required to be in a range of 2 times to 20 times the sectional diameter of the cylinder. The range, however, can be changed according to the size of a shielded space or to the intensity of a magnetic field inside the space. In the figure, a sectional view is shown in which a large diameter ferromagnetic cylindrical collar (2) and a small diameter ferromagnetic cylinderical collar(3) are disposed in the opening portion of the cylindrical magnetically shielding body (1) in a coaxial manner for the prevention of magnetic penetration. In the figure, there are shown a space G1 and a distance d1 between the magnetically shielding body (1) and the large diameter ferromagnetic cylindrical collar (2), a space G2, and a distance d2, between the large diameter ferromagnetic cylindrical collar (2) and the small diameter ferromagnetic cylindrical collar (3), and an internal space G3 of the small diameter ferromagnetic cylindrical collar (3) and a distance of radius R3 from the wall of the small diameter cylindrical collar (3) to the center axis. A part denoted with the numeral (4) is a magnetic field sensor placed in an objective feeble magnetic field space.
In the case where the superconductor cylinder (1) is applied alone, a penetrating magnetic field from the opening portion is expressed by following equations (I) and (II).
Equation (I) expresses the magnetic field intensity Ht when an external magnetic field is applied to the cylinder in the direction perpendicular to the cylinder axis (lateral magnetic field), and Equation (II) expresses the magnetic field intensity Ha when an external magnetic field is applied to the cylinder in the direction parallel to the cylinder axis (longitudinal magnetic field). R1 is the radius of the superconductor cylinder and Z is the distance from the opening end of the cylinder. From the above equations, it is known that the quantity of a penetrating magnetic field is larger when the external magnetic field is applied to the cylinder in the direction perpendicular to the cylinder axis, and when the quantity is to be reduced, for example, to 10xe2x88x928 times the external magnetic field, the objective space has to be positioned in the cylinder at a distance of 10 times the radius of the cylinder from the opening end portion. In other words, the length of the cylinder requires ten times the radius of the cylinder.
In the magnetically shielding structures according to preferred embodiment of the present invention, there are a mode in which cylindrical collar means made of a highly permeable material is disposed in the position within the range from the center portion to the end portion of the superconductor cylinder or disposed in the vicinity of the end portion of the cylinder body, a mode in which a plurality of cylindrical collar means of a highly permeable material are arranged in the longitudinal direction in the vicinity of the end portion, a mode in which a plurality of cylindrical collar means of a highly permeable material are disposed in multiple coaxial manner in the radial direction, etc. In the case, in particular, where a plurality of collars of a highly permeable material are arranged in the radial and/or longitudinal directions leaving spaces and/or intervals in multiple layers and/or in lamination, a magnetic field penetrating into the cylindrical superconductor shielding body can be decreased better than in the case where the single collar of a highly permeable material is used. The cylindrical collar of a highly permeable material of any shape can be used, so far as the collar can be inserted into the cylindrical shielding body of a superconductive material, and magnetic induction can be generated in the collar of a high permeability material and the penetrating magnetic field is absorbed in the highly permeable material and is magnetically shorted. The coller is, to be concrete, of a double open end type having a long cylinder length in comparison with the thickness. The shape can be a circle, an ellipse or a polygon. The collar can also be formed as a tapered cylindrical collar in which the inner diameter decreases with the increase in the distance from an end portion in the longitudinal direction or a cylindrical collar in which the shape of an inner cross section differs from that of an outer cross section having uneven places on the outer or inner surface. The shielding cylinder body can be formed as a cylindrical body in which the inner portion has a larger diameter in comparison with that of the end portion, a bellows-shaped cylindrical body, a cylinder of an L type, a T type, an H type, a cross type, a U type, or their combinations.
The ratio of the outer diameter of the collar of the highly permeable material to the inner diameter of a cylindrical shielding body composed of the superconductive material is shown in embodiments to be described later. When the collar of the high permeability material is used as a single piece and the collar having the outer diameter of more than ⅕ the inner diameter of the cylindrical shielding body is disposed inside the-superconductor cylinder body, a better shielding effect can be obtained than that when a superconductor cylinder is used alone. To be more precise, in order to shorten the length of the cylindrical shielding body, a method for calculating the attenuating quantity of a magnetic field penetrating through the gap between the superconductive body and the ferromagnetic collar is elucidated and the present invention is based on the result.
FIG. 2 is a diagram showing the calculation result of an attenuating quantity of a magnetic field penetrating through the gap between the superconductor body and the ferromagnetic collar. In the figure, the ordinate shows the K value of the exponent xe2x80x9cexp(xe2x88x92Kz/d) or Kxe2x80x2 value of xe2x80x9c10xe2x88x92Kxc2x7z/dxe2x80x9d, and the abscissa shows a relative diameter D of an inner cylindrical member, assuming that the diameter of an outer cylindrical member is 1, or the gap g between the inner and the outer cylindrical member. MSt denotes a magnetic field attenuation constant in the gap between cylindrical members for an external magnetic field (t) perpendicular to the cylinder axes of inside cylindrical ferromagnetic member (M) and the outside cylindrical superconductive member (S), and SMa denotes a magnetic field attenuation constant in the gap between both cylindrical members for an external magnetic field (a) in the direction of the axes of the inside cylindrical superconductive member (S) and the outside cylindrical ferromagnetic member (M).
In FIG. 1, G1 is a gap betwen the ferromagnetic member and the superconductive member. From FIG. 2, when d1 less than 0.5R1, the attenuation of a magnetic field in the gap G1 is approximately
When d2 less than 0.5R2, the attenuation of a magnetic field in the gap G2 is approximately
Ktxe2x80x2=Kaxe2x80x2=1.36.
The attenuation of a magnetic field in the gap G3 is approximately
When the diameter of the ferromagnetic cylindrical member is so designed that the magnetic fields penetrating into these three gaps are made equal to each other at an exit portion, that is, at Z=L, it is an optimum design for making the penetrating magnetic field a minimum. Therefore, the following relations,
0.62/d1=1.36/d2=2.4/R3
can be obtained. From these relations,
R2=0.860R1
and
xe2x80x83R3=0.548R1
can be obtained, and the magnetic field at the exit portion at the length L is as follows
H=10xe2x88x922.4 L/R3=10xe2x88x924.38 L/R3
The above value is remarkably improved in comparison with the value
H=10xe2x88x921.83 L/R3
which is obtained when the single superconductor cylinder body used alone. From the comparison of these equations, it is understood that the same attenuation quantity of magnetic fields can be obtained when the ratio between the lengths of cylindrical members is 1/5.54. In the above case, two sets of ferromagnetic collars are used, and when n sets of them are used the radii and gaps are
Rp=[3.76+1.36(nxe2x88x92p)]R1/(1.66+1.36 n),
d1=0.62R1/(1.66+1.36n)
and
dp=1.36 R1/(1.66+1.36n).
The attenuation of a magnetic field in this case is
H=10xe2x88x92(1.66+1.36 n) L/R1
When n is increased, the attenuation quantity of a magnetic field can be made large. There is, however, a limit because of the distances between them.
The clearance between the superconductive member and the ferromagnetic member is, in particular, limited because cooling is needed for the superconductive member.
In general, when a uniform lateral magnetic field Ho is applied to a single cylinder of a high permeability material having an infinite length, the internal magnetic field Hi is
xe2x80x83Hi=2rHo/xcexctxe2x80x83xe2x80x83(III)
where xcexc is permeability, t a thickness and r a radius, thus the attenuation quantity of a magnetic field has a finite value. In a feeble magnetic field, permeability of a ferromagnetic member is much decreased and the quantity of attenuation of a magnetic field by a ferromagnetic member is limited. The practical limit of attenuation is in the order of 10xe2x88x925. The value of n is decided in consideration of these points.
As described in the above, according to the present invention, against a penetrating magnetic field which shows an attenuating distribution toward the center on the longitudinal axis of a cylindrical shielding body composed of a material manifesting the Meissner effect, magnetic induction is generated in a cylindrical collar means of a highly permeable material, and, as a result, a penetrating magnetic field is absorbed in the highly permeable material and is magnetically shorted, resulting in further decrease in the magnetic field penetrating into the inside of the cylindrical shielding body of the superconductive material.
There are, to be concrete, shielding structures in which a cylindrical collar means of a highly permeable material is disposed inside a superconductor cylinder body or in the vicinity of the end portion of the body, a plurality of cylindrical collars of the highly permeable material are arranged in the longitudinal direction in the vicinity of the end portion, or a plurality of cylindrical collars of the highly permeable material are arranged in a coaxial manner in the radial direction. In particular, the shielding structures in which a plurality of cylindrical collars of a highly permeable material are arranged in the radial direction and/or longitudinal direction leaving spaces and/or intervals in multiple layer and/or in lamination, can better decrease a magnetic field penetrating into the inside of a cylindrical superconductor shielding body than those in which a single collar of a highly permeable cylinder is used. By adopting the shapes and dispositions as described in the above, a shielding effect, which is not possible to obtain with a shielding cylinder body alone, can be obtained a volume of a usable space of a feeble magnetic field intensity can be increased, and the length of the cylindrical superconductor shielding body for obtaining an objective magnetically shielded space or a space of feeble magnetic field intensity can be shortened, which makes it possible to decrease the cost of a shielding equipment.
The above and other features and advantages of the present invention will be made clearer in the explanations of the embodiments referring to the accompanying drawings, which are not intended to limit the scope of the present invention.